Membrane filters with corkscrew vortex generating means

ABSTRACT

A filter comprising a tubular filter membrane (1) within which is mounted a concentric core (2) formed with a helical groove (3) and leaving a gap (G). Fluid to be filtered is passed in the upwards direction into the space between the core (2) and the membrane (1) and thus constrained to move in a helical path within a helical space (8) defined between the groove (3) and membrane (1), with leakage flow (7) through the gap (G) between adjacent turns of the helix. The leakage flow (7) shapes and enhances an already-existing vortex flow within the space (8) and ensures effective scouring of the membrane to prevent build-up of solids on the membrane surface. A version incorporating an externally threaded helical groove is also described. The filter is useful for a wide range of filtration purposes, particularly in ultrafiltration and microfiltration.

This invention relates to membrane filters particularly, but notexclusively, of the cross-flow type.

The performance of membrane filters is limited by the efficiency offluid mixing to bring as much as possible of the fluid being filteredinto contact with the membrane.

Various solutions have been proposed. One involves pulsing the fluidwith an oscillatory flow along a channel at least partly defined by themembrane, the membrane either being furrowed or dimpled or the channelcontaining spaced obstructions, in order to produce eddies and othersecondary flows. A difficulty of this is in the complexity of themechanism required to produce the oscillatory flow. Other proposals haveinvolved the use of narrow channels but have required high pressuredrops from inlet to outlet, caused by frictional losses in the channels.Typical inlet and outlet pressures are 3.5 and 1 bar, respectively, andthese produce uneven filtration and demand high pumping power. A thirdapproach has been to use large bore tubes lined with thin layers ofmembrane which are perfused at flow rates high enough to ensureturbulent flow. This may solve the problem of poor mixing but at theexpense of high pumping power and possible damage to components, such asproteins or blood cells, in the feed fluid.

What is required is a membrane filter which uses laminar secondary flowsto provide gentle, efficient mixing, which can be scaled up easily andwhich requires modest pumping power.

An approach which goes some way to meeting these requirements is toinduce a force in a direction tending to throw the particles beingfiltered away from the membrane to thus prevent clogging of themembrane. In a tubular filter this may be achieved by defining a helicalpath along which the fluid being filtered may flow. Such an arrangementis described in U.S. Pat. No. 3,768,660 which discloses a reverseosmosis cell comprising an elongate hollow porous core having an osmoticmembrane on its surface. Fluid to be filtered is passed into the annulusformed between the core and an outer tubular shell. A flexible helicalmember is mounted within this annulus to thus define a helical path forthe fluid flowing in the annular space. The circular flow thus inducedcreates a centrifugal force which acts to keep particles away from themembrane, to prevent clogging.

The present invention seeks to provide a filter which utilizes helicalflow to induce centrifugal forces, but having improved performance byinducing a further component of motion in the fluid flowing around thehelix, namely a corkscrew vortex component, which has been found to givemuch improved flushing of the membrane surface whilst at the same timemaintaining substantially laminar flow to prevent damage to theparticulate components in the fluid being filtered.

According to the present invention, there is provided a filtercomprising a tubular, substantially cylindrical, porous membranearranged coaxially with, and radially spaced from, a generallycylindrical profiled surface, which surface is formed with at least onehelical groove, the arrangement being such that, in use, a fluid to befiltered is passed from one axial end along the passage defined betweenthe membrane and the profiled surface. The invention is characterised inthat means are further provided for inducing or enhancing, in thehelical flow of the fluid to be filtered, a corkscrew vortex flow. Itwill be understood that the word corkscrew is intended to give the ideaof a corkscrewing motion of the fluid in the existing direction of thehelical flow--in other words a further helical component of fluid motionin addition to the principal helical flow defined by the helical groove.Preferably the vortex flow is such as to substantially fill the wholecross section of the helical flow path of the fluid being filtered. Thisis achieved by careful design of the geometry of the tubular membraneand profiled surface. In particular, those parts of the profiled surfacewhere it most closely approaches the membrane between adjacent turns ofthe groove(s) are spaced from the membrane to define a narrow gap whichwill provide a leakage flow from turn to adjacent turn of the groove(s).The interaction between this leakage flow and the flow of the fluidalong the helical path provided between the groove(s) and membrane willproduce gentle laminar secondary flows, at least partly in the form ofeddies, thereby providing the desired mixing to bring the fluid intomaximum contact with the membrane. The gap is preferably at least tentimes smaller than the diameter of the tubular membrane.

The cross sectional shape of the helical groove is important inachieving satisfactory vortexing of the flowing fluid. Preferably theshape is continuously curved, for example elliptical or circular. Apart-circular cross section, in particular a semi-circular cross sectionhas been found to give good results, but in certain circumstances thiscould be modified as will be explained later. However, the groove shouldnot be too shallow since the vortex will be difficult to maintain andwill break up into individual vortices which in turn will lead torelative "dead" zones in the motion of the fluid being filtered. Forbest results the width of the helical groove, in the longitudinaldirection of the tubular membrane, is not significantly greater thanthree times its depth. Preferably the width of the groove is not greaterthan twice its depth.

The profiled surface may be formed as the outer peripheral surface of acore within the tubular membrane, or as the inner peripheral surface ofa shell surrounding the tubular membrane. Thus it will be seen that theprofiled surface has the form of an external or internal thread.Although only a single-start thread is described herein, the threadcould be multi-start, for example double-start or triple-start.

Mathematical models and preliminary experiments establish the efficiencyof the new system. The helical groove arrangement should be easy tomanufacture and is likely to find application in membrane separationprocesses where flow rates are sufficient to generate the necessarysecondary flows. In these applications the feed flow can be steady, thusavoiding problems associated with the complexity of the oscillatorydrive systems and difficulty of scaling up of the vortex mixing andstanding vortex wave designs previously developed. An obviousapplication of the invention would be to large scale ultrafiltration ofliquid food and beverages, where high pumping costs are a major problem.Other applications include microfiltration (membranes with porestypically of 0.2 μm diameter, for cell concentration); reverse osmosisfor the desalination of water, and gas exchange using a microporoushydrophobic membrane with pores of diameter of less than 0.02 μm.

In order that the invention may be better understood, severalembodiments thereof will now be described by way of example only andwith reference to the accompanying drawings in which.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are an axial section and a cross section respectivelythrough a first embodiment of a filter according to the invention;

FIG. 2 is an enlarged view of part of FIG. 1A;

FIGS. 3A and 3B are an axial section and a cross section respectivelythrough a second embodiment of a filter according to the invention;

FIG. 4 is an enlarged view of part of FIG. 3A;

FIGS. 5 and 6 are diagrams to illustrate the vortex flow in the crosssection of the helical groove;

FIGS. 7 and 8 are diagrams to show test circuits for oscillatory andchopped flow respectively applied to the filter of FIGS. 1 and 2;

FIG. 9 is a view of a test circuit to demonstrate microfiltration ofblood using the embodiment of FIGS. 3 and 4;

FIGS. 10 and 11 are graphs plotting plasma flux against channel pressurefor different geometries of filter;

FIG. 12 is a graph plotting plasma flux against Reynolds number usingthe test circuit of FIG. 9; and

FIG. 13 is a graph plotting power dissipation against Reynolds numberusing the test circuit of FIG. 9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The first filter shown in FIGS. 1 and 2 is primarily intended forseparating or concentrating proteins by molecular size. It consists of atubular ultrafiltration membrane 1 and a cylindrical concentric core 2formed on its exterior surface with a helical groove 3. The membrane hasvery small pores, for example of the order of 0.005 μm in diameter.

In use feed liquid is passed from one end to the other through the gapbetween the core and membrane. Fluid flow in the longitudinal directionof the filter is thus partly by a leakage flow 7 through a gap G definedbetween adjacent turns of the helical groove 3 and partly by a flow 9along the helical path presented by the helical space 8 bounded by thegroove 3 and the adjacent surface of the membrane 1. The leakage flow 7between adjacent grooves augments secondary vortex flows generated inthe helical space 8 by centrifugal forces. Making the cross section ofthe groove semicircular helps to maintain these secondary flows. Waterand salts are filtered through the membrane. A combination of high shearat the membrane wall and the strong secondary flows keep the fluid wellmixed, and thereby stops the retained proteins from blocking themembrane pores. The direction of filtrate flow is indicated by theletter F.

The filter shown in FIGS. 3 and 4 is primarily intended for separatingcellular components from a fluid, e.g. red cells from whole blood. Herethe helical space 8 is defined between a cylindrical shell 4 whoseinternal surface is formed with a helical groove 5, and a tubularmicrofiltration membrane 6 which surrounds, and is concentric with, theshell 4. In this case the membrane has larger pores, for example of theorder of 0.5 μm in diameter, that is much smaller than cell diameters.

In use, the feed liquid is passed from end to end through the passagebetween the shell and membrane. In this case centrifugal force acts topush the blood cells towards the outer wall while the plasma movestowards the inner (membrane) wall. The centrifugal force on the cellsexceeds the drag on the cells inwards by the plasma filtering throughthe membrane. The secondary flows are useful here, since otherwise cellswould move along the helical path only relatively slowly and would tendto clog the filter on the outer impermeable wall.

In both the internal and external embodiments described above, thegeometry of the arrangement is important in ensuring effectiveoperation. The object of the exercise is to enhance the secondary vortexflow in the fluid passing along the helical space 8 defined by thehelical groove 3 or 5. FIGS. 2 and 4 show the shape and position of thecomponents in enlarged detail.

It will first be noted that, as illustrated, the grooves 3,5 aresubstantially semicircular in section. Other shapes are possible, but itis important to maintain the smooth curved surface of the groove toprevent generation of unwanted local eddies. Thus, stated in generalterms, the groove should have a shape, when seen in longitudinal crosssection, that is generally concave and is,formed of a substantiallycontinuously curved surface, preferably with no breaks which might causeeddies. Although a circular shape is preferred, other shapes such aselliptical would be possible. The groove might usefully be deeper thanpure semicircular, but it should not be too much shallower. If, forexample, the width of the groove in the longitudinal direction is takenas b, and the depth of the groove, not including the gap G is taken asa, then the following inequality should apply:

    a<b<5a

but preferably

    b≈2a(i.e. semicircular)

The gap G is defined between the crests of the ridge between adjacentturns of the helical groove 3, 5, and the facing diameter of themembrane 1, 6. This gap is important in creating the leakage flow 7 fromone turn to the next. The width of the gap is such that it is between 10and 60 times smaller than that diameter d of the membrane surface whichfaces the helical groove. Preferably the gap is between 30 and 50 timessmaller than diameter d of the membrane.

The shape of the crest of the ridge is not felt to be of too muchsignificance, except that the downstream edge of the crest should bereasonably sharp in order to provide as abrupt as possible a change forleakage fluid entering the space 8 from the gap G. This will enhance thevortex effect by ensuring sudden separation of leakage flow at thedownstream edge of the crest.

FIGS. 5 and 6 illustrate the action of the leakage flow on the patternof flow in the space 8. FIGS. 5 and 6 illustrate just a single space 8,and represent a single turn of an internal helical groove of the typeshown in FIGS. 1 and 2; however, the same principles apply to theexternal groove of FIGS. 3 and 4. The upstream side of the filter is tothe left; the downstream side to the right. Thus leakage fluid entersthrough gap G on the left and exits to the right. FIGS. 5 and 6 havebeen prepared using mathematical modelling techniques and the density oflines is intended as a measure of the strength of the flow in thedirection indicated.

FIG. 5 shows the situation in which the gap G is, in effect, of zerosize so that the flow 7 of leakage fluid is likewise zero. The drawingrepresents the natural vortex flow pattern of fluid flowing around ahelical path. As can be seen, the flow pattern comprises a left and aright corkscrew vortex pattern of substantially equal strength andextent. These flow patterns are created by centrifugal forces which, inthe example illustrated, act substantially vertically in a directionfrom the bottom to the top of the drawing.

FIG. 6 shows the effect of applying a leakage flow 7. The action of thehigh velocity leakage flow is to overwhelm the pre-existing upstreamcorkscrew vortex and reinforce the downstream corkscrew vortex. Thus, inFIG. 6, it can be seen that the left-hand anticlockwise vortex isdiminished in both size and strength, whilst the right-hand vortex isenhanced in both size and strength. In perfect conditions, the left-handvortex will be reduced to zero and the flow through the filter will thuscomprise three components:

1) The leakage flow passing through gap G. This can comprise as much as50% of the bulk flow of fluid through the filter.

2) A helical flow within the space 8 bounded by the helical groove 3 or5 and the adjacent membrane surface, this flow being subject to acentrifugal force extending radially from the longitudinal central axisof the filter. This centrifugal force is either towards or away from themembrane according to which of the filter types, FIG. 1 or FIG. 3respectively, is being considered.

3) Also within the space 8, a secondary flow in the form of a corkscrewvortex. Of the two components of flow within the helical space 8(helical and vortex) the vortex flow can be the stronger and the moreeffective in scouring the surface of the membrane to prevent clogging.For this purpose, it is clear that a single strong vortex is preferableto the situation illustrated in FIG. 5, or even FIG. 6, where relative"dead" zones in which fluid is relatively stagnant can cause build up ofmaterial on the membrane surface. The action of the vortex is thustwofold: firstly to scour the surface of the membrane to keep it clearof entrained solids, second to retain those entrained solids insuspension so that they are carried out of the filter by the helicalcomponent of flow. For certain fluids to be filtered, e.g. blood, theseeffects can be enhanced by arranging the filter such that thecentrifugal forces due to the helical flow act away from the filtermembrane. For this reason the; embodiment of Figures 3/4 has been foundbetter for such fluids.

A still further geometrical factor which can affect operation is that ofthe pitch of the helical groove 3 or 5. It has been found that the ratioP/d, where p is the pitch, and d is the diameter of the membrane asdefined previously, should lie in the range 1/8 to 1/1, preferably inthe range 1/4 to 1/2. For multi-start threads, this ratio should bemultiplied by the appropriate figure: for example, for a double-startthread, the ratio is multiplied by 2; for a triple-start thread, theratio is multiplied by 3.

The performance of the two filters described above have been evaluatedin a series of tests, the results of some of which are now presented.The test circuits used for these tests are illustrated in FIGS. 7, 8 and9. The common components of these circuits are a closed circuitcomprising a reservoir 10 containing the test fluid to be filtered andincorporating a stirrer 11, a peristaltic roller pump 12 and the filter13 under test. Pressure at various points around the circuit is testedby a pressure gauge 14, for example a Budenberg gauge. The filter 13 maybe pressurised by a needle valve 15 connected in series with its output.Filtrate F falls by gravity into a vessel, shown diagrammatically underreference 16.

The tests are designed to be operated with quasi-steady laminar helicalflows and were carried out on ultrafilters and microfilters:

Ultrafilters

An ultrafiltration rig (FIGS. 7 and 8) was built with a filter 13comprising two PCI tubular membranes 1 (PCI Membrane Systems Limited,polysulphone PU 120 with molecular weight cutoff of 20 kD), arranged inparallel and connected at the top to form a U-shaped path, and with eachtube containing a concentric insert (not shown). Each tubular membranewas of internal diameter 12.5 mm and effective length 950 mm. The totalmembrane area was 746 cm².

Two different designs of concentric insert were evaluated. The firstconsisted of helical inserts of the type illustrated in FIG. 1 ofmaximum diameter 11.9 mm and with a helix of semicircular cross-sectionand pitch 5.5 mm. The second were uniform rods (i.e. with no helicalgroove) of 9.9 mm diameter.

Ultrafiltration performance was measured from bovine serum albumin (BSA)solutions of high concentration (60 g/l) under a wide range of operatingconditions.

The roller pump 12 was used to provide quasi-steady flow from thereservoir 10, through the U-shaped ultrafilter 13, back to thereservoir. The ultrafilter was pressurised by varying the needle valve15 in the outflow line. Pressure at the downstream end of theultrafilter was measured with a Budenberg gauge 14.

Oscillatory flow was generated by a pair of circular pistons 17 (FIG. 7)acting on pump bags at the open ends of the U-shaped tubular channel.

In the configuration shown in FIG. 8, a rotary valve 18 was used toswitch the flow from one membrane tube to the other alternately. Thevalve rotated at 74 rpm, as represented by arrow A, providing a pulsefrequency of flow to each tube of 148 rpm, or 2.47 Hz.

Fresh bovine serum albumin was mixed with distilled water to aconcentration of 60 g/l and the value of the pH of the solution wasrecorded. The pH had a mean value of 6.99 and a range of (6.92, 7.15).

Clean water flux was measured at the beginning and end of eachexperiment, for which fresh membrane tubes were used. Distilled waterwas pumped through the ultrafilter at a chosen rate and at a controlledpressure. All the measurements reported here were undertaken at atrans-membrane pressure of 2 bar, measured at the downstream end of theultrafilter. Ultrafiltrate was collected in a measuring cylinder 16 overa 1-minute interval. Two such measurements were made for each setting ofthe parameters.

For measurements of ultrafiltration rate from BSA solutions, thereservoir was filled with 0.75 l of BSA solution at 60 g/lconcentration, which was then pumped through the circuit until 300 ml ofprime water was displaced. The prime volume of the circuit, excludingthe reservoir but including the filter and tubing, was measured to be400 ml. Thus the true concentration of the BSA solution was 52.9 g/l .

Filtration rate was measured as a function of operating parameters.

Microfilters

Two designs of microfilter were built and evaluated in the separation offresh bovine blood. The first design was of the type illustrated inFIGS. 1 and 2, and consisted of a tubular microfiltration membrane 1(Gelman Versapor 0.45 μm pore diameter), supported by a rigid Perspexshell with longitudinal plasma collection channels machined in it. Themembrane unit has an internal diameter of 12.5 mm and a length of 20 cmto provide an active membrane area of 79 cm². Two shapes of concentrichelical insert 2 were evaluated. Both were loosely-fitting and were ofdiameter 11.9 mm, and had helices of semi-circular cross-section leavinga gap G of 0.3 mm width. One insert had a helix of 3.5 mm pitch, theother had a helix of 5.5 mm pitch; the width of the groove 3 in thelongitudinal direction was 3.0 mm and 5.0 mm respectively.

The second design was of the type illustrated in FIGS. 3 and 4 andconsisted of a tubular microfiltration membrane 6 (Gelman Supor 0.2 μmpore diameter or Gelman Versapor 0.45 μm pore diameter) supported on agrooved, cylindrical core (not shown). A helical flowpath external tothe tubular membrane was provided by the moulded epoxy shell 4. Thisexternal helix was of semi-circular cross-section and had a pitch of 3.5mm; the width of groove 5 was 3.00 mm.

The microfilters were tested in the circuit shown in FIG. 9. Afour-roller peristaltic pump 12 provided quasi-steady flow at rates of100 ml/min to 400 ml/min. An elevated reservoir 10 of bovine bloodensured that the trans-membrane pressure at the outlet of themicrofilter lay in the range 60-90 mmHg. The blood haematocrit in thereservoir was maintained at 38%. Plasma flow rate was measured with acylinder and stopwatch.

Ultrafiltration

Filtration results, from high-concentration BSA (60 g/l), obtained withthe ultrafilter with rod inserts of 9.9 mm diameter are shown inTable 1. The type of flow applied (oscillatory, chopped or steady) isshown in the first column, the mean inlet flow rate is shown in thesecond column and the filtration rate in the third. Correspondingresults with the same filter perfused with clean, distilled water at thebeginning and end of the experiment are shown in the last two rows ofthe table.

                  TABLE 1                                                         ______________________________________                                        Operating                    R.sub.T bar                                                                         Rbar                                       Conditions                                                                             Q.sub.in ml/min                                                                         Q.sub.F ml/min                                                                          cm/s  cm/s  R.sub.m /R.sub.T                     ______________________________________                                        BSA 60g/                                                                      Oscillatory,                                                                           144       15.4      5806  3921  0.32                                 low flow                                                                      Oscillatory,                                                                           405       16.8      5322  3437  0.35                                 high flow                                                                     Chopped, 145       7.0       12789 10904 0.15                                 low flow                                                                      Chopped, 338       10.2      8755  6870  0.22                                 high flow                                                                     Steady,  159       9.6       9339  7454  0.20                                 low flow                                                                      Steady,  399       13.5      6654  4769  0.28                                 high flow                                                                     Clean Water                                                                   Beginning                                                                              383.5     47.5      1885  --    1.0                                  End      365.5     45.5      1967  --    0.96                                 ______________________________________                                    

The results are presented in terms of total resistance, R_(T) (bar cm/s)in the fourth column. The fifth column gives the resistance R, after themembrane resistance R_(m), is deducted. The efficiency, R_(m) /R_(T), isshown in the sixth column.

The resistances were calculated as follows:

The filtration flux, J_(T) was calculated from ##EQU1##

Since the effective membrane area was 746 cm².

The total resistance R_(T) is given by ##EQU2## where R_(m) is themembrane resistance (measured by clean water flux), R is the fluid-sideresistance and Δ p is the trans-membrane pressure in bar.

The ratio of the filtration fluxes (BSA and clean water) is ##EQU3##

The corresponding results for the tubular rig with the helical insertsof FIG. 1/2 are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                        Operating                    R.sub.T bar                                                                         Rbar                                       Conditions                                                                             Q.sub.in ml/min                                                                         Q.sub.F ml/min                                                                          cm/s  cm/s  R.sub.m /R.sub.T                     ______________________________________                                        BSA 60g/                                                                      Oscillatory,                                                                           131.5     44.4      2016  511   0.75                                 low flow                                                                      Oscillatory,                                                                           378.5     44.5      2012  507   0.75                                 high flow                                                                     Chopped, 153       28.3      3169  1664  0.47                                 low flow                                                                      Chopped, 388.5     36.5      2453  948   0.61                                 high flow                                                                     Steady,  148       32.1      2786  1281  0.54                                 low flow                                                                      Steady,  375       44.3      2020  515   0.75                                 high flow                                                                     Clean Water                                                                   Beginning                                                                              367.5     59.5      1505  --    1.0                                  End      391       55.0      1628  --    0.92                                 ______________________________________                                    

It can be seen that the results for the helical filter are much betterthan for the filter with the 9.9 mm diameter bare rods. Results with 7.9mm diameter bare-rod inserts were even worse, and are not presentedhere. Perhaps the most striking result is that steady flow through thehelical filter provided 54% (low flow) and 75% (high flow) of the fluxrate achieved with clean water at the same transmembrane pressure.

Neither chopped nor oscillatory flow produced any further enhancement inefficiency at the higher flow rate through the helical filter.

Microfiltration

Plasma flux (i.e. plasma filtration rate divided by membrane area) isshown plotted against channel pressure drop, as measured by pressuregauges 14 at input and output, in both of FIGS. 10 and 11. FIG. 10presents three sets of results, namely 3.5 mm pitch and 5.5 mm pitchinternal helical members of the type illustrated in FIGS. 1 and 2 and5.5 mm pitch external helical members of the type illustrated in FIGS. 3and 4. The modest results achieved with the internal helical member, inparticular, prompted further investigation of filters using externalhelical members of the type described with reference to FIGS. 3 and 4.

The external helix microfilter with 5.5 mm pitch was evaluated withbovine blood using two different polysulphone membranes. Filtrationperformance for two different tests are shown in FIGS. 11 and 12. TheReynolds number is given by ##EQU4## where U_(peak) is the velocitycalculated by dividing the outlet blood flow rate, Q_(out) by thecross-sectional area of the helix. The channel gap is defined as theradius of the helix cross-section, and is given by 2h =1.5 mm for thehelix of 3.5 mm pitch and 2h=2.5 mm for the helix of 5.5 mm pitch.

The external helix microfilter produced a flux of around 0.1 cm/min whenthe Supor membrane was used, double the performance achieved with theinternal helix microfilter. When the Versapor membrane was used (whichdiffered from the Supor by having a stiff fibrous support structure aswell as slightly larger pores), the results were even better, giving aflux of around 0.125 cm/min. This matched the best of the results wehave obtained with vortex-mixing and standing-vortex-wave microfilters,and was achieved at a matching peak Reynolds number.

There was no evidence of red cell damage in any of the blood filtrationexperiments.

Power dissipation

The power dissipation within the external helix can be calculated fromthe product of the pressure drop over the 20 cm length, Δ p, and theblood flow rate Q_(out) :

    P=ΔpQ.sub.out

The results presented in FIG. 13 show that the superior filtrationperformance of the Versapor membrane is counter-balanced by the greaterpower dissipation. For both Supor and Versapor a curve-fit shows thatthe power dissipation is approximately proportional to the square of theReynolds number. This can be interpreted as a linear relationshipbetween pressure drop and Reynolds number which in turn suggests laminarflow conditions.

Effect of Pump Frequency and Pulsations

we assessed the influence of the peristaltic pump by replacing thestandard 4 mm bore tubing with a larger tube of 8 mm bore. This allowedus to maintain our range of flow rates at lower rotational pumpfrequencies. The performance is compared at two Reynolds numbers inTable 1.

The results show that higher rotational speeds enhance filtration flowrates. This suggests that flow pulsations aid the mixing process andenhance filtration efficiency.

                  TABLE 3                                                         ______________________________________                                        Reynolds                                                                             Pump Frequency                                                                             Tubing Diameter                                                                             Plasma Flux                                 Number rpm          mm            cm/min                                      ______________________________________                                        162    20           8             0.081                                       162    80           4             0.108                                       238    30           8             0.089                                       238    110          4             0.142                                       ______________________________________                                    

It is felt that careful design of the flow waveform applied to thefilter could augment the effects already achieved by the designdescribed above. A sharp deceleration in flow rate enhances the vortexeffect and therefore a sawtooth-like waveform, with a slow acceleration,followed by a sharp deceleration could be beneficial. The relationshipbetween the pulse rate and the pitch of the helical groove 3, 5 may alsobe significant. In particular, the following inequality should apply:##EQU5## where frequency is in Hz pitch is in m

velocity is in m/sec

Discussion of Results

The tubular ultrafilter with helical inserts worked very well whenperfused with quasi-steady flow at modest flow rates (400 ml/min). Evenat high concentrations of bovine serum albumin (60 g/l), filtration fluxreached a level of 75% of the maximum possible for the tubularmembranes.

The tubular microfilter with similar helical inserts also workedreasonably well, providing plasma flux of between 0.045 and 0.055cm/min, although plasma flux in the range 0.1 to 0.15 cm/min isroutinely obtained with standing-vortex-wave microfilter designs.However, these require oscillatory flow.

The tubular microfilter with an external helix of 5.5 mm pitch producedplasma flux of up to 0.15 cm/min, matching the best of the previousdesigns, but using quasi-steady flow. The improved performance of theexternal helix design, compared with the filter with an internal helicalinsert appears to be due mainly to the centrifugal forces acting to movethe blood cells away from the membrane in the external helix design, andacting to move the cells towards the membrane in the internal helixdesign.

The helical designs will be easy to manufacture and are likely to findapplication in membrane separation processes where flow rates aresufficient to generate the necessary secondary flows. In theseapplications, which will include ultrafiltration, microfiltration andgas exchange, the feed flow can be steady.

I claim:
 1. A filter comprising a tubular, substantially cylindrical,porous membrane mounted coaxially with and radially spaced from agenerally cylindrical profiled surface, which surface is formed with atleast one helical groove such that a fluid to be filtered is passed intothe filter from one end thereof between the membrane and the profiledsurface in a helical flow, and means for inducing or enhancing, in thehelical flow of the fluid to be filtered, a corkscrew vortex flow, saidmeans comprising the shape of said groove, when seen in longitudinalcross section, being concave and formed of a substantially continuouslycurved surface.
 2. A filter as claimed in claim 1 wherein said means aresuch as to produce a single corkscrew vortex within the helical flow,said vortex substantially filling the whole space occupied by saidhelical flow, when seen in longitudinal section.
 3. A filter as claimedin claims 1 or 2 wherein the profiled surface has a diameter such as todefine a gap between itself and the membrane, in which gap a leakageflow of fluid occurs between adjacent turns of the helical groove, saidleakage flow being operable to induce or enhance said corkscrew vortexflow within the helical flow.
 4. A filter as claimed in claim 3 whereinthe width G of said gap is at least 10 times smaller than the facingdiameter of the tubular membrane.
 5. A filter as claimed in claim 4wherein said gap G is between 10 and 60 times smaller than the facingdiameter of the tubular membrane.
 6. A filter as claimed in claim 5wherein said gap G is between 30 and 50 times smaller than the facingdiameter of the tubular membrane.
 7. A filter as claimed in claim 1 or 2wherein the shape of the groove in cross section is part-circular.
 8. Afilter as claimed in claim 1 or 2 wherein the shape of the groove incross section is semi-circular.
 9. A filter as claimed in claim 1 or 2wherein the width of the helical groove in the longitudinal direction isnot greater than three times the depth of the groove.
 10. A filter asclaimed in claim 9 wherein the width of the groove in the longitudinaldirection is not greater than twice the depth of the groove.
 11. Amethod of filtering a fluid comprising: constraining the fluid beingfiltered to flow along a helical path bounded at least in part by aporous filter membrane and an opposing profiled surface; inducing orenhancing a corkscrew vortex flow in the helical flow; and selectivelyconfiguring the shape of the profiled surface and the location of theprofiled surface relative to the filter membrane such that the helicalflow is subject to a single corkscrew vortex flow which is dominant interms of size and strength and acts on substantially the whole helicalflow.
 12. A method as claimed in claim 11 wherein a leakage flow offluid being filtered is created between adjacent turns of said helicalpath, said leakage flow acting to induce or enhance said corkscrewvortex flow.
 13. A filter comprising a tubular, substantiallycylindrical, porous membrane mounted coaxially with and radially spacedfrom a generally cylindrical profiled surface, said profiled surfacehaving a diameter such as to define a gap between itself and themembrane, and which surface is formed with at least one helical groove,the shape of said groove, when seen in longitudinal cross section, beingconcave and formed of a substantially continuously curved surface, suchthat a fluid to be filtered is passed into the filter from one endthereof between the membrane and the profiled surface and flows in ahelical path, said gap being such as to cause a leakage flow of fluidbetween adjacent turns of the helical groove, said leakage flow beingoperable to induce or enhance, within the helical flow of the fluid tobe filtered, a corkscrew vortex flow.
 14. A filter as claimed in claims1,2 or 13 wherein, for a single-start helical groove, the ratio P/d,where p is the pitch of the helical groove, and d is the facing diameterof the membrane, is in the range 1/8 to 1/1.
 15. A filter as claimed inclaim 14 wherein the ratio P/d is in the range 1/4 to 1/2.
 16. A filteras claimed in claims 1, 2 or 13 wherein the profiled surface is formedas the outer peripheral surface of a cylindrical core mounted coaxiallywithin the tubular membrane.
 17. A filter as claimed in claims 1, 2 or13 wherein the profiled surface is formed as the inner peripheralsurface of a cylindrical shell within which the membrane is mountedcoaxially.